Shrinking crude oil reserves, limited refining capacity, concern about environmental protection and greenhouse gas effects have led to renewed interest in alternative fuels, lubricants and greases. In the last decade a new twist on an old idea, updated technologies are allowing researchers to develop bio-based lubricants that are more stable (towards oxidation and cold temperature) than the predecessors. These new oils, lubricants, greases and industrial fluids are derived from common plant products such as soybean, sunflower and canola (or in Europe, its close relative rapeseed) oil by transesterification of triglycerides with alcohols. Advantages of biolubricants include that they are renewable, biodegradable, less environmentally hazardous and safer to human contact unlike the petroleum-based lubricants and oils. Biolubricants are fatty acid alkyl esters having 22 to 26 carbon atoms. In recent years, for reasons ranging from religious, ethical and health issues there is a growing tendency towards using vegetable-based products in many formulations. Thus apart from its use in automobile industry, biolubricants find applications also in the manufacture of cosmetics, feedstuffs, soaps etc.
Several processes for transesterification of triglycerides have been developed: (1) Base-catalyzed transesterification of oil with alcohol (catalysts—alkaline metal alkoxides and hydroxides as well as sodium and potassium carbonates), (2) Direct acid-catalyzed esterification of the oil with alcohol (catalysts—Brönsted acids, preferably sulfonic acid and sulfuric acid), and (3) Conversion of oil to fatty acids and then to alkyl esters with acid catalysis. However, the former route (i.e., base-catalyzed reaction) is the most economical and in fact, is in practice in several countries for bio-diesel production (J. Braz. Chem. Soc. Vol. 9, No. 1, Year 1998, pages 199-210; J. Am. Oil. Chem. Soc. Vol. 77, No. 12, Year 2000, pages 1263-1266; Fuel Vol. 77, No. 12, year 1998, pages 1389-1391; Bioresource Tech. Vol. 92, Year 2004, pages 55-64; Bioresource Tech. Vol. 92, Year 2004, pages 297-305; Renewable Sustainable Engery Rev. Vol. 9, Year 2005, pages 363-378). Alkaline metal alkoxides (as CH3ONa for the methanolysis) are the most active catalysts, since they give very high yields (>98%) of fatty acid alkly esters in short reaction times (30 min) even if they are applied at low molar concentrations (0.5 mol %) (J. Food Composition and Analysis Year 2000, Vol. 13, pages 33-343). However, they require high quality oil and the absence of water, which makes them inappropriate for typical industrial processes (J. Braz. Chem. Soc. Vol. 9, No. 1, Year 1998, pages 199-210). Alkaline metal hydroxides (NaOH and KOH) are cheaper than metal alkoxides but require increasing catalyst concentration (1-2 mol %). NaOH is more superior to KOH as the latter and other alkali hydroxides yield more saponified products than the bio-fuel.
Recently, enzymatic transesterification using lipase has become more attractive for bio-fuel production, since the glycerol produced as a by-product can easily be recovered and the purification of fatty acid esters is relatively simple to accomplish. However, the main hurdle to commercialize this system is the cost of lipase production (J. Mol. Catal. B: Enzymatic Vol. 17, Year 2002, pages 133-142).
Use of immobilized lipases in the synthesis of fatty acid methyl esters from sunflower and soybean oils were reported by Soumanou and Bornscheuer and Watanabe et al (Enzy. Microbiol. Tech. Vol. 33, Year 2003, page 97; J. Mol. Catal. B: Enzymatic Vol. 17, Year 202, pages 151-155). They found that the immobilized enzyme is active at least for 120 h during five batch runs without significant loss of activity. Among the various lipases investigated the enzyme from Pseudomonas fluorescens (Amano AK) exhibited the highest conversion of oil. Khare and Nakajima (Food Chem. Vol. 68, Year 2000, pages 153-157) also reported the use of immobilized lipase enzyme.
Cost is the major factor slowing the commercialization of biofuels. Replacement of homogeneous by a solid catalyst eliminates the processing costs associated with the homogeneous catalysts. At the end of the reaction, the solid catalyst can be recovered by simple filtration from the product mixture and reused. Corma et al. (J. Catal. Vol. 173, Year 1998, page 315) evaluated transesterification of triglycerides with glycerol using basic solid catalysts such as Cs-MCM-41, Cs-Sepiolite and hydrotalcite. The reaction had to be carried out at 240° C. for 5 h to achieve high conversions. Hydrotalcite gave good conversion of 92% followed by Cs-sepiolite (45%) and Cs-MCM-41 (26%). Leclercq et al. (J. Am. Oil. Chem. Soc. Vol 78, Year 2001, page 1161) studied the transesterification of rapeseed oil in the presence of Cs-exchanged NaX and commercial hydrotalcite (KW2200) catalysts. At a high methanol to oil ratio of 275 and 22 h reaction time at methanol reflux, the Cs-exchanged NaX gave a conversion of 70% whereas 34% conversion was obtained over hydrotalcite. ETS-4 and ETS-10 catalysts gave conversions of 85.7% and 52.7%, respectively at 220° C. and 1.5 h reaction time (U.S. Pat. No. 5,508,457). Suppes et al (J. Am. Oil. Chem. Soc. Vol. 78, Year 2001, page 139) achieved a conversion of 78% at 240° C. and >95% at 160° C. using calcium carbonate rock as catalyst. Of late, Suppes et al reported the use of Na, K and Cs exchanged zeolite X, ETS-10, NaX occluded with NaOx and sodium azide in the transesterification of soybean oil with methanol (Appl. Catal. A: Gen. Vol. 257, Year 2004, page 213). Furuta et al (Catal. Commun. Vol. 5, Year 2004, pages 721-723) describe biodiesel production from soybean oil and methanol at 200-300° C. using solid superacid catalysts of sulfated tin and zirconium oxides with oil conversions over 90%. Use of tin complexes immobilized in ionic liquids for vegetable oil alcoholysis was reported by Abreu et al (J. Mol. Catal. A: Chem. Vol. 227, Year 2005, pages 263-267; J. Mol. Catal. A: Chem. Vol. 209, Year 2004, pages 29-33). Kim et al reported the use of heterogeneous base catalysts (Na/NaOH/Al2O3) for the methanolysis of vegetable oils
U.S. Pat. No. 5,713,965 describes the production of biodiesel, lubricants and fuel and lubricant additives by traneseterification of triglycerides with short chain alcohols in the present of an organic solvent such as an alkane, arene, chlorinated solvent, or petroleum ether using Mucor miehei or Candida Antarctica-derived lipase catalyst. Patents Nos. WO 00/05327 A1, WO 02/28811 A1, WO 2004/048311 A1, WO 2005/021697 A1 and WO 2005/016560 A1 and U.S. Pat. Nos. 6,642,399, 6,399,800 also teach us the production fatty acid alkyl esters using either lipase catalysts or homogeneous acid/base catalysts. Patent No. WO 2004/085583 A1 describes transesterification of fats with methanol and ethanol in the presence of a solid acid catalyst having ultrastrong-acid properties in a short time at around ordinary pressure.
Most of the reports in the open literature are confined to preparation of fatty acid methyl or propyl esters for biodiesel applications. Studies on transesterification with long chain alcohol are scarce. Production of bio-diesel from pure soybean oil or coconut oil is not economical, so it is desirable to use cheaper alternative feedstocks such as animal fat or used cooked oil or oil from seeds of wild plants like jojoba and jatropha. Animal fat and used oil contain high amounts of free fatty acids (FFA) content. The FFA saponifies with the alkali-based transesterification catalyst leading to low yield, difficulties in separation of the products, and increase in production cost. In those cases a two step process wherein in the first step an acid catalyst esterifies the free fatty acids to methyl esters and in the second step a base catalyst transesterifies the triglycerides is generally employed in biolubricants preparation. An efficient solid catalyst, which can do this in a single-step is more advantageous. Thus more efficient catalysts that could circumvent the above drawbacks of the prior-art processes in lubricants preparation are highly desirable.
The present invention deals with a process, which eliminates most of the above said drawbacks. It deals with production of lubricants which comprises reacting of vegetable oils or fats with long chain alcohols say for example C6 to C8 alcohols at moderate conditions using a novel, solid, reusable double metal cyanide catalyst. The feedstock vegetable oil is a pure triglyceride or a mixture of fatty acids and tryglycerides. One of the metals of the double metal cyanide catalyst is Zn2+ while the other is Fe. Co-existence of Zn and Fe in the active site linking through cyano bridges makes it efficient to transform feedstocks containing fatty acids in a single step to fatty acid esters. The catalyst could be separated easily by centrifugation or by simple filtration and reused. Most importantly, the catalyst is highly efficient and only a small amount (1 wt % of oil) is needed to carryout the reaction. The process is atom-efficient and the reaction conditions like temperature and pressure are only moderate. Unlike the conventional base catalysts the catalyst of the present invention is more efficient even in the presence of water impurity in oil. Hence, there are no limitations on the quality of oil that should be used with the present catalysts.